Differentiate between competing star-formation histories for RUBIES-EGS-QG-1

Determine whether the stellar population of the quiescent galaxy RUBIES-EGS-QG-1 at redshift z=4.90 has a low metallicity of approximately 0.2 Z_sun with an early formation redshift z_form ≈ 10 and quenching by z ≳ 8.6, or instead has solar metallicity with a later formation redshift z_form ≈ 6.3 and quenching by z ≈ 5.7, given that current data do not robustly distinguish between these two star-formation histories.

Background

The authors model the JWST/NIRSpec PRISM spectrum and JWST/HST photometry of RUBIES-EGS-QG-1 with Prospector, finding that the galaxy is massive and quiescent, with deep Balmer absorption lines indicating rapid quenching. However, the inferred stellar ages depend sensitively on the assumed metallicity.

Their fiducial fit with metallicity as a free parameter favors a low stellar metallicity (~0.2 Z_sun), implying very early formation (z_form ≈ 10) and quenching by z ≳ 8.6. In contrast, a model with fixed solar metallicity yields a substantially younger population (z_form ≈ 6.3) and more recent quenching (z ≈ 5.7). With the current data, the authors cannot robustly discriminate between these two scenarios, although they note the stellar mass and the burst and quenching timescales are relatively insensitive to metallicity.